I was thinking of a beacon, not much bandwidth needed there...
There isn't much information from an ETI that would make any sense to us anyway, except a simple numerical progression just complex enough to confirm the transmission was not natural in origin and was composed by a mathematician.
1...2...3...4...5...6
or squares
1...4...9...16...25...36
or primes
1...2...3...5...7...11...13
or even the Fibonacci sequence
0, 1, 1, 2, 3, 5, 8, 13...F(i) = F(i-1) + F(i-2)